Problem: Which of these numbers is prime? ${23,\ 62,\ 77,\ 81,\ 87}$
A prime number has exactly two factors: 1 and itself. The factors of 23 are 1 and 23. The factors of 62 are 1, 2, 31, and 62. The factors of 77 are 1, 7, 11, and 77. The factors of 81 are 1, 3, 9, 27, and 81. The factors of 87 are 1, 3, 29, and 87. Thus, 23 is a prime number.